The present invention relates to the technical field of methods for recirculating a reprocessing effluent comprising chloride ions, in particular a reprocessing brine for an anion exchange resin intended for discoloration of a colored sweetened solution.
Preferably, the reprocessing effluent comprises a reprocessing brine and coloring agents from the treatment of the colored sweetened solution. These coloring agents notably comprise polyphenols.
Ion exchange resins in particular of the strong anionic resin type in the form of a chloride are widely used for discoloring syrups, i.e. solutions comprising one or several sugars in sugar refineries. The binding mechanisms of the coloring agents on ion exchange resins are multiple and notably apply ion exchange between certain coloring agents, having an organic acid nature, and the chloride ions from the ion exchange resin, as well as the adsorption of hydrophobic coloring agents on the matrix of the resin. After saturation of the ion exchange resin, the reprocessing of this resin is carried out by percolating a volume of salted water of a solution of a salt at a relatively high concentration, in particular at 100 g/l, at a pH comprised between 11 and 13. This salt solution is also called a reprocessing brine.
Several mechanisms come into play upon reprocessing the ion exchange resin with the reprocessing brine, an osmotic shock is notably produced, the internal water of the resin beads flows out of the beads so as to dilute the reprocessing brine. Ion exchanges, solvation of the coloring agents, decrease in the matrix/coloring agents hydrophobic interaction are also observed.
Only a minor fraction of the chloride ions contained in the reprocessing brine i.e. from about 5% to 10% of the applied reprocessing brine, is actually exchanged with the resin by ion exchange or displacement of a coloring agent/organic acid equivalent of the sweetened solution colored by a chloride ion equivalent.
At the outlet of the column comprising the ion exchange resin, about 95% of the chlorides of the applied salt for reprocessing are thus found in the reprocessing eluates, also described as reprocessing effluents.
These eluates are more diluted than the initial reprocessing brine and optionally loaded with coloring agents. It is thus not possible to directly recover a reprocessing brine ready for use from the reprocessing effluent because of the dilution of the chloride ions.
Indeed, the efficiency of the reprocessing depends on the concentration of chloride ions in the reprocessing brine, which is preferably around 100 g/l.
These reprocessing effluents, loaded with salt, and optionally with coloring agents, in particular polyphenols, are highly pollutant since they are very difficult to degrade as such. The coloring agents are however degradable by a biological treatment provided they have been separated from the salt.
Because of the difference in size between the salt, i.e. the chloride ions, and the macromolecules which the coloring agents are, the separation of both of these species by applying a nanofiltration membrane is quite easy.
Such a method was applied at the Marseille sugar factory since 1998. In this case, the most concentrated fraction of the reprocessing effluent is selected, this fraction for which the salt titer is of the order of 80 g/l of salt represents about 80% to 90% of the salt contained in the eluates. This fraction is treated by nanofiltration. By this nanofiltration, the coloring agents are concentrated 10 to 15 times in the nanofiltration retentate from this fraction highly concentrated in salt. The majority of the salt actually passes through the nanofiltration membrane, and is thus again found in the nanofiltration permeate which will be used as a base for the next reprocessing of the ion exchange resin.
The concentration of chloride ions of the nanofiltration permeate went up to 80 to 100 g/l by addition of fresh brine concentrated at 250 g/l, the pH is adjusted.
This nanofiltration permeate thus having its titration of chloride ions, readjusted, may again be used as a reprocessing brine.
Advantageously, with this simple method, 80% of the salt is thus recycled. The fraction of the nanofiltration retentate rich in coloring agents is mixed with the diluted fractions of the reprocessing effluent so as to be treated in a water treatment plant. This method however has the drawback that a large fraction of the reprocessing effluent comprising a low concentration of chloride ions, in particularly clearly less than 80 g/l and notably of the order of 10 to 20 g/l, is lost. The provision of salt, corresponding to about 20% of the needs, is brought by adding fresh brine directly concentrated into the permeate so as to adjust the reprocessing brine in volume and in concentration.
Moreover, there exist increasingly restrictive standards as regards the discharges in effluents and therefore a demand for limiting at most the volume of effluents to be treated.
In certain countries, in particular in those in which water is rare, it is imperative to limit at most the volume of effluents to be treated and to recycle a maximum of water. In order to meet these goals, a method is notably known, consisting of collecting the totality of the fractions of the reprocessing effluent in order to separate the coloring agents from the salt by nanofiltration.
In this case, the fractions loaded with salt and colored of the reprocessing effluent are collected so as to be subject to nanofiltration for separating the coloring agents from the salt. The amount of thereby collected salt represents almost the whole of the salt contained in the eluate but its concentration in the nanofiltration permeate is lower since it is of about 50 g/l of chloride ions. The adjustment of the volume and of the concentration of the regenerating substance requires a concentration step by evaporation of the water of the collected fractions in order to adjust the salt concentration. This step may be directly carried out in an air heater; in this case the evaporated water is lost, the use of an evaporator gives the possibility of recovering a portion of the condensates which may be reused as processed water. Regardless of the concentration technique, this process is costly in energy since it assumes strong consumption of steam for evaporating a large amount of water. When an evaporator is applied, the condensation of the water vapors requires the application of a cooling tower; the recovery of water is therefore not complete. It is thus estimated that the water consumption required for condensation of the emitted vapors is of the order of ⅓ of the produced condensates. Further, conducting the air heaters and the cooling tower is sometimes delicate because of the dispersion of aerosols often deemed to be at the origin of sanitary problems. Finally, the risk of corrosion induced by the concentration of a salt solution imposes the use of expensive special acids for building this evaporator.